1. Field of the Invention
The present invention relates generally to electronic signal measurement apparatus, and more particularly to a system for detection and acquisition of automotive fuel injector voltage signal pulse width regardless of the fuel injector type.
2. Description of the Prior Art
In the automotive repair field, as well as in other fields, it has long been important to have instruments available for measuring electrical signals occurring at various points within the numerous electrical circuits and signal paths resident in an automobile. Measurements of parameters such as current, voltage, resistance, signal frequency, etc. enable a repair technician to locate and diagnose the numerous problems that occur in a vehicle. Such parameters are typically measured using available apparatus ranging from simple voltage, current and resistance-measuring meters to sophisticated, computerized electronic diagnostic equipment.
A signal of particular interest is the voltage signal of a fuel injector, more specifically, the pulse width of the voltage signal as detected at the fuel injector. Fuel injectors of an automobile receives periodic voltage pulses of a duration as specified by the computer module of the automobile. In one operation, the computer module receives a signal from the oxygen sensor indicating the amount of oxygen remaining after combustion of the air-fuel mixture. In response to such a signal, the computer module adjusts the amount of fuel to be injected by the injectors by varying the pulse width of the voltage signal sent to the fuel injectors.
There are generally two activation methods for fuel injectors, feed-side controlled and ground-side controlled. FIGS. 1a-1d illustrate four feed-side controlled pulse types that may be detected at the fuel injector voltage terminals. The ground symbols serves as a reference point for the depicted voltage signals. In FIG. 1a, a Port Fuel Injector (PFI) type pulse 10 comprising of a base pulse 12 and a kickback pulse 14 is illustrated. FIG. 1b illustrates a "peak & hold" or "current limited" pulse 16 comprising of a base pulse 18 and two kickback pulses as indicated at 20 and 22. FIG. 1c illustrates a Modulated with One Kickback pulse 24 comprising of one base pulse 26 immediately followed by two (or more) shorter pulses (28 and 30) and a kickback pulse 32. In FIG. 1d, a Modulated with Two Kickback pulse 34 is comprised of a base pulse 36 followed by a kickback pulse 38, two (or more) shorter pulses (40 and 42), and another kickback pulse 44. Note that the kickback pulses typically are much larger than the base pulses and they are illustrated with broken dots to emphasize the voltage differences.
FIGS. 4a-4d illustrate four ground-side pulse types that may be detected at a fuel injector. In FIG. 4a, there is a PFI type pulse 46 with a downward base pulse 48 followed by a kickback pulse 50. In FIG. 4b, there is a "peak & hold" or "limited current" pulse 52 comprising of a base pulse 54 followed by two kickback pulses (56 and 58). FIG. 4c illustrates a Modulated with One Kickback pulse 60 with a base pulse 62 followed immediately by two (or more) shorter pulses (64 and 66) and a kickback pulse 68. In FIG. 4d, a Modulated with Two Kickback pulse 70 is comprised of a base pulse 72 followed by a kickback pulse 74, two (or more) shorter pulses (76 and 78) and another kickback pulse 80. All of the detected kickback pulses are byproducts of the operation of the fuel injector coil and are further explained below.
The number of `shorter` base pulses on all modulated injectors (1 or 2 kickbacks, feed-side or ground-side) varies with the duration of the total injector event. The duration of the first (wider) base pulse is relatively fixed, and that is the amount of time it takes for the current flowing into the injector to actuate the pintle. To maintain the pintle in the actuated state allowing a greater amount of fuel injected, shorter base pulses are provided. The figures of the voltage signals of the moduled injectors (FIGS. 1c, 1d, 4c, and 4d) depict the waveforms at one particular pulse width (around 2.5 ms). At a short pulse width (around 2 ms), there may only be one shorter pulse; at a longer pulse width (around 5 ms), there may be 6 or 7 shorter pulses. To summarize, the wider base pulse is generally fixed, and the number of shorter base pulses increases or decreases to adjust the total pulse width.
In operating a feed-side controlled injector, a positive voltage signal is applied by the vehicle's computer module which causes a current to flow through the injector coil to produce a magnetic field that activates a pintle to allow fuel to be injected through a valve opening into the combustion chamber of a cylinder. The pulse width of the voltage signal corresponds to the duration or amount of time the pintle remains actuated and therefore the amount of fuel injected into the cylinder. If a high level of oxygen remains after combustion (indicating a `lean` air-fuel mixture), the oxygen sensor detects and reports such a condition to the computer module and the computer module in response increases the pulse width of the voltage signal sent to the fuel injector and thereby increases the amount of fuel injected into the cylinder. If an undesirably low level of oxygen is detected and reported by the oxygen sensor (indicating a `rich` air-fuel mixture condition), the computer module in response to such condition decreases the duration of the voltage signal to decrease the amount of fuel injected into the cylinder. Upon turning off the voltage to the injector coil (after the falling edge of the pulse), there is a kickback pulse having a magnitude far exceeding the magnitude of the base pulse due to the collapsing magnetic field within the injector.
Ground-side controlled injectors operate in likewise manner as that of the feed-side controlled injectors with the difference being that the injectors are supplied with constant voltage by the vehicle's electrical system, and the computer module supplies or removes a ground path to the injector to control current flow. When a ground path is supplied to the injector, the pintle is actuated to allow fuel to be injected into the cylinder.
In addition to the signal from the oxygen sensor, the computer module receives other input signals that may cause the computer module to adjust the pulse width of the voltage signal to the fuel injectors in response to the input signals. Thus, by varying an input signal to the computer module known to cause the computer module to adjust the pulse width of the voltage signal to the fuel injectors and by observing the detected voltage signal at a fuel injector, the operation of a number of subsystems can be diagnosed for proper operation.
Prior art measurement systems for observation of injector voltage pulse width modulation proves to be difficult to use or inaccurate. Such systems include oscilloscopes, digital multimeters with capability for measuring pulse width, and digital multimeters with special capability for measuring fuel injector pulse width. For an oscilloscope, the user must manually synchronize the voltage signal from the injector and manually measure and calculate the start and stop times of the pulse event in order to obtain the pulse width. This is a very time consuming process prone to mistakes in the measurement of the pulses.
For digital multimeters featuring capability for measuring pulse width, the measured and displayed values are generally incorrect due to the fact that this type of multimeter typically uses a single-point voltage reference. The pulse width of a fuel injector voltage signal generally comprises one or more base pulses and one or more inductor kickback pulses. A single-point voltage reference measures either the base pulse width or the kickback pulse width but not both, and is therefore unable to correctly detect the true pulse width.
Prior art digital multimeters featuring capability for measuring fuel injector pulse width require the user to first identify whether the injector is a feed-side controlled or ground side controlled injector before connecting the probes of the multimeter to the injector. This additional step of identifying the injector type prior to the using of the multimeter hinders efficient diagnosis of an automobile and requires the user to have a higher knowledge level of the fuel injector system. Additionally, some of these multimeters use pattern matching to identify the pulse type. However, pattern matching methods are unable to detect new pulse types that may be later developed by the automobile manufacturers.